This invention relates to electrostatography. More particularly, this invention relates to corona generating devices for applying electrostatic charge onto a suitable surface and means for maintaining the corona wires of such devices in a taut or tensioned condition during operation.
In the electrostatographic process, an electrostatographic plate comprising a photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity which reaches them and thereby creates an electrostatic latent image on or in the plate coating which may then be developed into a visible form by applying a developer material to the plate using any one of a number of development techniques generally known and used in the art. The developer material electrostatically clings to the plate in a visual pattern corresponding to the electrostatic image. Thereafter, the developed image is usually transferred from the plate to a support material, such as paper, to which it may be fixed by any suitable means thereby forming a permanent print.
The charging of the electrostatographic plate in preparation for the exposure step is accomplished by means of a corona generating device whereby electrostatic charge is applied to the electrostatographic plate to raise it to a potential in the range of approximately 600-1000 volts. Examples of corona generating devices for this purpose are disclosed in U.S. Pat. Nos. 2,777,957 and 2,836,725. In U.S. Pat. No. 2,777,957 a plurality of parallel wires are connected to a high voltage source and supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. When the wires are energized, corona is generated along the surface of the wires and ions are caused to be deposited on the adjacent photoconductive surface. Suitable means are usually provided to effect relative movement between the surface to be charged and the corona generating device. Such a device may alternatively have a single corona wire, as shown in U.S. Pat. No. 2,836,725.
It has heretofore been established that consistent high quality reproductions can best be obtained when a uniform potential is applied to the electrostatographic plate in preparation of the plate for exposure step. If the electrostatographic plate is not charged to a sufficient potential, the electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of developer material thereon will be correspondingly small. If, however, the electrostatographic plate is overcharged, the converse will occur and if overcharged sufficiently, the photoconductive layer of the electrostatographic plate can be permanently damaged. Also, since the contrast value of the electrostatic latent image is related directly to the potential charge on the electrostatographic plate before exposure, it is apparent that if the plate is not uniformly charged over its entire area, the contrast value of the electrostatic latent image obtained upon exposure will vary in different areas on the plate, and an uneven or mottled effect will be visible on the image when developed.
It is therefore important that the coronodes of single or multiple corona generating devices which are stretched between mountings at opposite ends of the device be maintained in taut condition, since slackness and kinks in these coronode wires will result in non-uniformity of the charge applied to the electrostatographic plate.